1. Field of the Invention
The present invention relates to a monofilament which decomposes in the natural environment, which is excellent in heat stability and forming workability, which has high mechanical strength, particularly knot strength and longitudinal cracking resistance, and which is suitable for products typified by strings for tennis rackets.
2. Description of the Related Art
In the recent years, from the viewpoint of natural environmental protection, a biodegradable polymer and its processed product, which decompose in the natural environment, are required and studies on naturally degradable resins such as an aliphatic polyester are done actively. In particular, one of these examples, a polylactic acid, potentially has a heat of combustion not more than half of that of polyethylene, and is naturally hydrolyzed under the ground or in water and thereafter converted by microorganisms into harmless decomposed products. Studies for obtaining formed articles, specifically films, sheets, fibers and the like, using polylactic acid are being made. Polylactic acid can be improved in its strength by stretching. However, since polylactic acid is a hard and brittle material, it is poor in flexibility and usability and also in longitudinal cracking resistance and knot strength. Polylactic acid is not practical, therefore.
On the other hand, an aliphatic polyester composed of a polycondensation product from an aliphatic polyfunctional carboxylic acid and an aliphatic polyfunctional alcohol is an example of flexible, naturally degradable resins.
For example, Japanese Patent Publication (B2) No. 2851478 reports that a monofilament excellent in heat stability and mechanical properties can be obtained by using, as a biodegradable monofilament, an aliphatic polyester which has been prepared by increasing, with a coupling agent, a molecular weight of a relatively high molecular weight polyester prepolymer having a hydroxyl group on terminal of molecule which mainly contains a polyester obtained by reacting two components of a glycol and a polybasic acid (or anhydride thereof), and if necessary as a third component, at least one polyfunctional component selected from tri- or tetra-functional polyhydric alcohols, oxycarboxylic acids and polyvalent carboxylic acids (or anhydride thereof).
Japanese Laid-open Patent Publication No. 10-110332 (1998) reports that a monofilament suitable for a fishline can be obtained by melt-spinning a blend polymer containing at least one species of polyalkylene dicarboxylate having a melting point of not lower than 70xc2x0 C. as a first component and at least one species selected from polylactic acid, a copolymer of polylactic acid and a poly(xcex2-hydroxyalkanoate) as a second component in the range of (the first component)/ (the second component) weight ratio of 95/5-40/60.
However, the aliphatic polyesters obtained by these methods are flexible and low in tensile strength and exhibit great elongation, in general.. Therefore, there are problems on their practical use as monofilaments such as strings for rackets or fishlines. Moreover, since they contain much materials of relatively low melting points, they may cause problems of fusion due to frictional heat and scuffing due to rubbing, and therefore are not practical.
The strings for rackets means strings to be used for rackets for tennis, soft tennis, badminton, squash and racquetball.
Heretofore, a string for a racket has been called gut. Gut is literally made of gut of sheep or cow and has still been used by some players typified by professional players at present because of its overall playability feeling and the like. Particularly in soft tennis, whale tissue has been used in stead of gut for tennis and occupies a position similar to gut.
These have advantages in ball-hitting sounds, creep property and the like in addition to overall playability feeling, but also have disadvantages of poor water resistance, high price and the like.
For these reasons, strings made of synthetic fibers, particularly polyamide-type synthetic fibers, are now in the mainstream in the world. This is because the problems of low water resistance and high price, which are drawbacks of natural gut, can be overcome and they are excellent in durability when repeatedly used.
By the way, overall playability, durability and ease of stringing are three important properties required of strings for rackets. Lacking any one of these properties breaks their practical usability.
The overall playability, which is mainly resilience at hitting ball, includes feelings such as ball-holding feeling and soft feeling following vibration, and ball-hitting sounds. The durability includes both durability against wear in repeated use and retentiveness of tension of a string face. The ease of stringing is ease to install a string to a racket.
It has been well known that when aliphatic polyesters relating to the present invention as well as aromatic polyesters are applied for strings for rackets, good overall playability can be obtained as predicted from their tenacity-elongation curves. However, there are two problems about durability, that is, problems on the face tension retentiveness and on longitudinal cracking of filament which takes place at the time of tightening a string with a clamp or the like when the string is installed to a racket. That is, this means that there has been two problems in the important characteristics.
Thinking that overcoming these two problems can make the polyesters excellent materials as a material for strings for rackets in association with their biodegradability, the present inventors have made intensive studies and have accomplished the present invention.
The object of the present invention is to provide a monofilament which has sufficient mechanical strength to be practically used also as a string for rackets and which is excellent in workability.
The present inventors eagerly studied in order to solve such problems, and have found that the above object can be accomplished by extruding a material prepared by blending mainly a polylactic acid type polymer and an aliphatic polyester other than polylactic acid in a predetermined proportion and drawing the extruded material.
That is, the present invention is a monofilament formed by extruding a material prepared by blending mainly a polylactic acid type polymer and an aliphatic polyester other than polylactic acid and drawing the extruded material.
Moreover, the present invention is a monofilament formed by extruding a material prepared by blending mainly a polylactic acid type polymer and an aliphatic polyesters other than polylactic acid in a blending ratio in weight of 95/5-61/39 and drawing the extruded material.
Furthermore, the present invention includes:
the monofilament wherein at least two species of the aliphatic polyesters other than polylactic acid are contained;
the monofilament wherein the aliphatic polyesters other than polylactic acid are mainly composed of an aliphatic carboxylic acid component and an aliphatic alcohol component, and/or an aliphatic hydroxycarboxylic acid component;
the monofilament wherein the polylactic acid and the aliphatic polyester other than polylactic acid have a crosslinked structure formed either in each of and/or between them;
the monofilament wherein the melting point of the aliphatic polyester other than polylactic acid is lower than that of the polylactic acid type polymer;
the monofilament wherein the drawing was conducted at temperatures not lower than the melting point of the aliphatic polyester other than polylactic acid;
the monofilament wherein the polylactic acid type polymer is oriented and the aliphatic polyester other than polylactic acid is not oriented;
the monofilament wherein the final draw ratio is 3-9 times; and
the monofilaments wherein surfaces of the aforementioned monofilaments are covered with a polymer material such as an aliphatic polyester and a polyurethane.
Furthermore, the present invention is a process for producing a monofilament comprising extruding a material prepared by blending mainly a polylactic acid type polymer and an aliphatic polyesters other than polylactic acid in a blending ratio in weight of 95/5-61/39, and drawing the extruded material at a temperature not lower than the melting point of the aliphatic polyester other than polylactic acid.
Furthermore, the present invention includes:
the process for producing a monofilament wherein at least two species of the aliphatic polyester other than polylactic acid are used; and
the process for producing a monofilament wherein the polylactic acid type polymer and the aliphatic polyester other than polylactic acid have a crosslinked structure formed either in each of and/or between them.
Moreover, the present invention includes strings for rackets, sutures, fishlines or strings for musical instruments consisting of these monofilaments and also includes strings for rackets, sutures, fishlines or strings for musical instruments using these monofilaments as a part of their structural members. The strings for rackets include strings to be used for rackets for tennis, soft tennis, badminton, squash and racquetball etc.
The polylactic acid, aliphatic polyester other than polylactic acid and the like to be used in the present invention are explained step by step below.
In the present invention, a polylactic acid is a polymer composed substantially only of monomer units derived from L-lactic acid and/or D-lactic acid. The term xe2x80x9csubstantiallyxe2x80x9d used herein means that the polylactic acid may contain other monomer units derived from neither L-lactic acid nor D-lactic acid unless the effect of the present invention is impaired.
As a method for producing the polylactic acid can be adopted arbitrary known polymerization methods. The most typically known method is one in which a lactide, which is an anhydrous cyclic dimer of lactic acid, is subjected to ring-opening polymerization (a lactide method), but lactic acid may be directly subjected to condensation polymerization.
In the case where the polylactic acid is composed only of monomer units derived from L-lactic acid and/or D-lactic acid, the polymer is crystalline and of high melting point. Furthermore, since the crystallinity and melting point thereof can be freely adjusted by changing a ratio of monomer units derived from L-lactic acid to those derived from D-lactic acid (abbreviated as an L/D ratio), a practical characteristic can be controlled depending upon uses.
Moreover, other hydroxycarboxylic acids or the like may be copolymerized unless the properties of polylactic acid are damaged.
Furthermore, for the purpose of increase in molecular weight or improvement in melt viscosity, a small amount of a chain elongating agent, a crosslinking agent and the like, such as diisocyanate compounds, epoxy compounds, acid anhydrides and peroxides, can be employed. A weight average molecular weight of the polymer is preferably in the range of 50,000 to 1,000,000. When it is less than this range, satisfactory mechanical properties and the like are not shown. When it exceeds that range, workability becomes poor.
In the present invention, the aliphatic polyester other than polylactic acid (henceforth, simply referred to as xe2x80x9caliphatic polyesterxe2x80x9d) includes, for example, a polymer composed of an aliphatic carboxylic acid component and an aliphatic alcohol component and a polymer composed of an aliphatic hydroxycarboxylic acid component.
Examples of methods for producing the aliphatic polyester include a method wherein a high molecular weight product is obtained by directly polymerizing these components, and a indirect method wherein a high molecular weight product is obtained by polymerizing these components so as to form an oligomer and then using a chain elongation agent or the like.
An example of the aliphatic polyester to be used in the present invention is an aliphatic polyester made from a dicarboxylic acid and a diol. The aliphatic dicarboxylic acid may be compounds including succinic acid, adipic acid, suberic acid, sebacic acid and dodecanoic acid, or anhydrides or derivatives thereof.
On the other hand, the aliphatic diol can be generally exemplified by glycol-type compounds such as ethylene glycol, butanediol, hexanediol, octanediol and cyclohexanedimethanol, and derivatives thereof. Any of them are compounds having an alkylene, cyclic, or cycloalkylene group having 2 to 10 carbon atoms, which can be produced by condensation polymerization. For both of the carboxylic acid component and the alcohol component, two or more species may be used.
Moreover, for the purpose of forming a branch in a polymer in order to improve its melt viscosity, tri or more-functional polyvalent carboxylic acids, polyhydric alcohols or hydroxycarboxylic acids also may be used. When these components are used in large amounts, the resulting polymers may have crosslinked structures to lose thermoplasticity, or to form microgels partially having highly crosslinked structures even if the polymers are thermoplastic. Proportions of these tri or more-functional components contained in the polymers, therefore, are very small and they are contained in degrees such that chemical and physical properties of the polymers are not influenced very much. As such a polyfunctional components, malic acid, tartaric acid, citric acid, trimellitic acid, pyromellitic acid, pentaerythit or trimethylol propane can be used.
Among the producing methods, the direct polymerization method is a method in which the above-mentioned compounds are chosen and a high molecular weight product is obtained with removal of moisture which has been contained in the compounds or is generated during the polymerization.
The indirect polymerization method includes a method comprising choosing and polymerizing the above-mentioned compounds so as to form an oligomer and thereafter growing the oligomer to have a high molecular weight by using a small amount of a chain elongating agent, for example, a diisocyanate compound such as hexamethylene diisocyanate, isophorone diisocyanate, xylylene diisocyanate, diphenylmethane diisocyanate for the purpose of increasing a molecular weight, and a method wherein an aliphatic polyester carbonate is obtained by using a carbonate compound.
The other aliphatic polyesters to be used in the present invention include, for example, an aliphatic polyester composed of a condensation polymerization product of an aliphatic hydroxycarboxylic acid. Examples of the aliphatic hydroxycarboxylic acid include glycolic acid, xcex2-hydroxybutyric acid, hydroxypivalic acid and hydroxyvaleric acid. Their condensation polymerization provides high molecular weight products. These aliphatic hydroxycarboxylic acids can be used in the form of their derivatives such as ester and cyclic ester. Ring-opening polymerization of the cyclic esters also gives high molecular weight products.
Moreover, when containing two or more kinds of aliphatic polyesters other than polylactic acid, the monofilament has characteristics excellent in knot strength and longitudinal cracking resistance. In particular, when the monofilament contains a polycaprolactone, the knot strength is more improved.
Preferable blending ratios by weight of the polylactic acid to the aliphatic polyester other than polylactic acid are 95/5 to 61/39. If the polylactic acid occupies 95% by weight or more, the knot strength can not be improved. On the other hand, if the polylactic acid is 61% by weight or less, defectiveness, such as difficulty in stringing when it is used as string, occurs because needed strength can not be achieved and also elongation is great.
Furthermore, since the polylactic acid and the aliphatic polyester other than polylactic acid have a crosslinked structure formed either in each of and/or between them, a heat resistant characteristic is improved, and even when a racket strung with string is left in a car in midsummer, the string is never cut. In addition, it is also important that reduction in face tension of string after stringing can be controlled more efficiently. Examples of available methods for introducing such a crosslinked structure to the polylactic acid and the aliphatic polyester other than polylactic acid include heretofore known methods e.g., a method comprising addition of tri or more-functional isocyanate compounds, epoxy compounds and acid anhydrides, a method using a radical generating agent such as peroxides, and a method comprising strong ultraviolet irradiation.
Next, a process for producing the monofilament of the present invention is explained.
First, although a method and a device for blending the polylactic acid and the aliphatic polyester are not particularly limited, ones which can work continuously are industrially advantageous and preferable.
For example, it is permitted that two or more kinds of pellets and various additives are blended in a predetermined proportion and are charged into a hopper of an extruding machine as they are, and thereafter they are molten and immediately formed into a monofilament. Moreover, it is also permitted that these ingredients are melt blended and thereafter formed into pellets in an intermediate step, and then the resulting pellets are molten and formed into a monofilament as needed.
Furthermore, it is also possible that the polylactic acid, the aliphatic polyester and the like are molten by an extruder or the like separately from each other, these are mixed in a predetermined proportion with a stationary mixer and/or a mechanical stirrer and immediately formed into a monofilament. Alternatively, the mixed material may be pelletized in an intermediate step. Mixing by mechanical stirring with an extruder or the like and a stationary mixer may be combined.
In order for uniform mixing, a method in which the mixture is pelletized in an intermediate step is preferable. But in the case of melt blending, substantial prevention from deterioration and degeneration of polymers and a copolymerization reaction due to a transesterification is required. Therefore, mixing is preferably conducted at a temperature as low as possible in a time as short as possible.
A melt extrusion temperature is selected adequately in consideration of melting points and a blending ratio of the resins to be used, but it generally ranges 100 to 250xc2x0 C.
The monofilament of the present invention is generally circular in section, but may be in a hollow shape in which its core portion is vacant, or in a deformed shape such as a diamond shape and a star shape. A diameter of the monofilament is not particularly limited and may be determined depending on uses intended. For example, when it is used as a string for a racket, it may be set to approximately 0.6 mm to 1.60 mm.
When the aforementioned polylactic acid and aliphatic polyester other than polylactic acid are used in order for forming the monofilament according to the present invention, modification can be carried out by adding transesterification catalysts, various monomers, coupling agents, terminal treating agents, other resins, wood meal, starch and the like in addition to various additives such as modifires, fillers such as calcium carbonate, lubricants, ultraviolet absorbers, antioxidants, stabilizers, pigments, colorants, various fillers, antistatic agents, mold release agents, plasticizers, perfumes and antibacterial agents, as needed. Furthermore, if we do not stick to biodegradability, other general purpose polymers and the like may be added.
It is also important in the present invention that the drawing after extrusion forming is conducted at temperatures not lower than the melting point of the aliphatic polyester other than polylactic acid.
That is, when the drawing temperature is not higher than the melting point of the aliphatic polyester other than polylactic acid, the aliphatic polyester component other than polylactic acid is also drawn and oriented, and therefore satisfactory knot strength and longitudinal cracking strength can not be achieved.
The drawing is conducted by providing a wet drawing chamber and a dry heat drawing chamber having a far-infrared heater, an electric heater or the like as a heat source between rollers driven at different rotation speed, or by heating a non-drawn monofilament by transmitting heat from a heating roller located on the supplying side, and by setting a predetermined speed ratio between rollers. Although the speed ratio between rollers, that is, a draw ratio, is approximately 4 to 10 times in the present invention, it is more preferably 3 to 9 times in consideration of a balance between longitudinal cracking and strength due to the drawing orientation.
In the present invention, characteristics excellent in tenacity and elongation can be achieved by orienting the polylactic acid type polymer, and at the same time, both the knot strength and the longitudinal cracking resistance can be satisfied simultaneously by not orienting the aliphatic polyester other than polylactic acid.
A tensile tenacity-elongation curve of the monofilament of the present invention is similar to that of natural gut such as sheep casing and whale tissue and the monofilament can achieve an overall playability feeling similar to that of natural gut. This is also one of the features of the monofilament of the present invention.
Moreover, covering the surface of the aforementioned monofilament with a polymer material can provide luster to a string surface to enhance its appearance, and also can improve durability of string. The polymer material for covering is preferably an aliphatic polyester particularly in consideration of biodegradability. But if we do not stick to the biodegradability, covering is preferably conducted by using a variety of elastomers such as polyurethane from the viewpoint of durability of string and the like. Covering with mixtures of these resins are also available.